Self-alignment for redistribution layer

ABSTRACT

An apparatus comprising a substrate with multiple electronic devices. An interconnect structure formed on a first side of the substrate interconnects the electronic devices. Dummy TSVs each extend through the substrate and form an alignment mark on a second side of the substrate. Functional TSVs each extend through the substrate and electrically connect to the electronic devices. A redistribution layer (RDL) formed on the second side of the substrate interconnects ones of the dummy TSVs with ones of the functional TSVs. Step heights of the RDL over the functional TSVs are less than a predetermined value, whereas step heights of the RDL over the dummy TSVs are greater than the predetermined value.

BACKGROUND OF THE DISCLOSURE

Two semiconductor wafers, dies, and/or other substrates may be assembledutilizing through-silicon vias (TSVs) that extend through the thicknessof the substrate. The top surface of at least one of the substratescomprises circuitry formed therein and/or thereon, whereas bond pads anda redistribution layer (RDL) are formed on the bottom surface todistribute power and signals to the circuitry through the TSVs.

A conventional RDL may comprise copper with an ENEPIG (electrolessnickel-electroless palladium-immersion gold) surface finish. However, assuch processing became quite expensive, a proposed alternative entailedincorporating an RDL comprising aluminum instead of copper. Nonetheless,shortcomings also exist with this alternative.

For example, the patterning utilized in forming an RDL comprisingaluminum requires additional processes to generate alignment marksutilized during lithography, such as to ensure a certain step heightafter the TSV formation for alignment marks. However, this step heightintroduces seam issues in the aluminum film, such as may be attributableto poor step coverage attainable with physical-vapor deposition (PVD) ofthe aluminum film.

SUMMARY OF THE DISCLOSURE

The present disclosure introduces an apparatus comprising a substratecomprising a plurality of electronic devices. An interconnect structureformed on a first side of the substrate interconnects ones of theplurality of electronic devices. A plurality of dummy TSVs each extendthrough the substrate and form an alignment mark on a second side of thesubstrate. A plurality of functional TSVs each extend through thesubstrate and electrically connect to one of the plurality of electronicdevices. A redistribution layer (RDL) formed on the second side of thesubstrate interconnects ones of the dummy TSVS with ones of thefunctional TSVs. Step heights of the RDL over the functional TSVs areless than a predetermined value, whereas step heights of the RDL overthe dummy TSVs are greater than the predetermined value.

The present disclosure also introduces a method comprising forming aplurality of TSVs each extending from a first surface of a substrate andprotruding from a second surface of the substrate, wherein the substrateis defined between the first and second surfaces, and wherein theplurality of TSVs comprises: a plurality of densely located dummy TSVs;and a plurality of spaced apart functional TSVs. A dielectric layer isformed over the second surface of the substrate and ends of each of theplurality of TSVs protruding from the second surface. Planarizing thenremoves the dielectric layer from over the dummy and functional TSVs, aswell as a substantial portion of each of the protruding ends of thefunctional TSVs. A redistribution layer is then formed over portions ofthe remaining dielectric layer between ones of the plurality of TSVs, aswell as ends of ones of the plurality of TSVs revealed by theplanarizing.

The present disclosure also introduces an apparatus comprising asubstrate comprising a plurality of electronic devices, and aninterconnect structure formed on a first side of the substrate andinterconnecting ones of the plurality of electronic devices. A pluralityof first through-silicon-vias (TSVs) each extend through the substrateand are electrically connected to one of the plurality of electronicdevices within a die area surrounded by a die street. A plurality ofsecond TSVs each extend through the substrate and form an alignment markwithin the die street. A redistribution layer (RDL) formed on the secondside of the substrate interconnects ones of the pluralities of first andsecond TSVs.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a sectional view of at least a portion of apparatus accordingto one or more aspects of the present disclosure;

FIG. 2 is a sectional view of the apparatus shown in FIG. 1 in asubsequent stage of manufacture according to one or more aspects of thepresent disclosure;

FIG. 3 is a sectional view of the apparatus shown in FIG. 2 in asubsequent stage of manufacture according to one or more aspects of thepresent disclosure;

FIG. 4 is a sectional view of the apparatus shown in FIG. 3 in asubsequent stage of manufacture according to one or more aspects of thepresent disclosure;

FIG. 5 is a sectional view of the apparatus shown in FIG. 4 in asubsequent stage of manufacture according to one or more aspects of thepresent disclosure;

FIG. 6 is a sectional view of the apparatus shown in FIG. 5 in asubsequent stage of manufacture according to one or more aspects of thepresent disclosure;

FIG. 7 is a plan view of the apparatus shown in FIG. 6 according to oneor more aspects of the present disclosure; and

FIG. 8 is a sectional view of the apparatus shown in FIG. 6 in asubsequent stage of manufacture according to one or more aspects of thepresent disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a first feature over or on a second featurein the description that follows may include embodiments in which thefirst and second features are formed in direct contact, and may alsoinclude embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

FIG. 1 is a cross-sectional view of a first semiconductor wafer 100having a front side 102 and a back side 104. The first semiconductorwafer 100 comprises multiple semiconductor dies each defined within acorresponding die area 106 demarcated by multiple die streets 108. Eachdie comprises a substrate 110, one or more active devices 120, aninterconnect structure 130, contact pads 140, under-bump metallization(UBM) 145, contact bumps 150, and functional TSVs 160. The substrate 110may comprise doped or undoped bulk silicon, or an active layer of asilicon-on-insulator (SOI) substrate, such as may comprise a layer of asemiconductor material (e.g., silicon, germanium, silicon germanium,and/or others) on an insulator layer. The substrate 110 may alsocomprise one or more multi-layered substrates, gradient substrates, orhybrid orientation substrates, among others.

The active devices 120 are represented on FIG. 1 as a single transistor.However, a wide variety of transistors, capacitors, resistors,inductors, and/or other active devices may be utilized to generate thedesired structural and functional requirements of the design. The activedevices 120 may be formed by any suitable methods, whether within and/oron the surface of the substrate 110.

The functional TSVs 160 may be formed by applying and developing asuitable photoresist (not shown), and then etching the substrate 110 togenerate TSV openings. Such openings may extend into the substrate 110at least further than the active devices 120, perhaps to a depth atleast greater than the eventual desired height of the finishedsemiconductor die. While the depth is dependent upon the overall designof the semiconductor die, the depth may range between about 1 micronsand about 700 microns below the surface of the substrate 110, such as toa depth of about 50 microns. The openings for the functional TSVs 160may have a diameter ranging between about 1 micron and about 100microns, such as a diameter of about 12 microns.

The openings for the functional TSVs 160 may be filled with a barrierlayer 162 and a conductive material 164. The barrier layer 162 maycomprise one or more layers of conductive and/or dielectric materials,such as titanium, titanium nitride, tantalum nitride, titanium, siliconnitride, silicon oxide, and/or other materials, which may be formedutilizing chemical-vapor deposition (CVD), plasma-enhanced CVD (PECVD),metal organic CVD (MOCVD), sputtering, and/or other processes. Theconductive material 164 may comprise copper, aluminum, dopedpolysilicon, combinations thereof, and/or other materials, and may beformed by depositing a seed layer and then electroplating copper ontothe seed layer. Grinding, chemical mechanical planarizing (CMP), and/orother processes may then be utilized to remove a portion of the barrierlayer 162 and/or the conductive material 164.

The interconnect structure 130 comprises multiple metallization layers132 and vias 134 connecting the various active devices 120 to formfunctional circuitry. The metallization layers 132 and vias 134 may beformed in alternating layers of dielectric and conductive material byany suitable processing, such as may comprise deposition, damascene,dual damascene, and/or other processes. The interconnect structure 130may comprise at least four metallization layers 132, which may beseparated from the substrate 110 by at least one interlayer dielectriclayer (ILD), although the precise number of metallization layers 132 mayvary dependent upon the design of the semiconductor die.

The contact pads 140 may connect the interconnect structure 130 toexterior input/output connections, such as the UBM 145. The contact pads140 may comprise aluminum, copper, alloys and/or combinations of these,and/or other materials, and may be formed in a variety of methodsdepending upon, for example, the selected composition. For example,patterning a layer of aluminum utilizing photolithography, chemicaletching, and/or other techniques may form aluminum contact pads 140.Alternatively, copper contact pads 140 may be formed in openings in adielectric layer 142 by depositing one or more barrier and/or seedlayers (not shown) in the openings, filling the openings with copper,and then planarizing.

The UBMs 145 may act as an intermediary between the contact pads 140 andthe contact bumps 150. The UBMs 145 may comprise a series of conforminglayers of conductive materials, such as a series of chrome,chrome-copper alloy, copper, and perhaps gold layers, a series oftitanium, titanium tungsten, and copper layers, and/or a series ofcopper, nickel, and gold layers, among other examples. Each such layermay be formed utilizing CVD, PECVD, sputtering, evaporation, plating,and/or other processes, perhaps to a thickness ranging between about 10microns and about 100 microns, such as to a thickness of about 45microns. One or more portions of such layers may then be removed byphotolithographic masking and etching processes, among others, leavingthe patterned UBMs 107.

The contact bumps 150 may comprise tin, silver, copper, and/or othermaterials. For example, tin solder contact bumps 150 may be formed byinitially forming a layer of tin through evaporation, electroplating,printing, solder transfer, ball placement, and/or other processing,followed by one or more reflow processes. The contact bumps 150 may havea thickness of about 100 microns. In some embodiments, the contact bumps150 are copper pillar bumps which may be formed by photolithography andelectro plating processes.

Scribe lines 170 define the borders between the die streets 108 and thedie areas 106. The die streets 108 are “formed” by not formingfunctional structures (such as active devices 120) between the scribelines 170 on opposing sides of each die street 108. Other structures areformed in the die streets 108, however, such as test pads, dummy metalsutilized for planarization, the dummy TSVs 210 described below, and/orother features that are not needed for the functioning of thesemiconductor dies 106 once the semiconductor dies 106 have been cutfrom the first semiconductor wafer 100. The die streets 108 may have awidth ranging between about 20 microns and about 180 microns, such as awidth of about 80 microns.

A person having ordinary skill in the art will readily recognize thatthe above description is merely illustrative and should not be viewed aslimiting the scope of the present disclosure to just the embodimentshown in FIG. 1. Numerous variations in the initial structure of thesemiconductor die in each die area 106 may also or alternatively beutilized. For example, other structures within the scope of the presentdisclosure may include those in which the functional TSVs 160 extend notonly through the substrate 110, but are also formed to extend throughone or more of the metallization layers 132.

FIG. 2 is a cross-sectional view of a portion of the apparatus shown inFIG. 1 in a subsequent stage of manufacture, in which the firstsemiconductor wafer 100 has been thinned to expose the functional TSVs160, and also to expose dummy TSVs 210 that were formed in the diestreets 108 simultaneously with the formation of functional TSVs 160.Such thinning may entail a multi-step, wet-etch process, such as maycomprise one or more bulk removal etching processes and/or one or morefinishing etching processes. The one or more bulk removal etchingprocesses may utilize a mixture of one or more of nitric acid (e.g.,HNO3), sulfuric acid (H2SO4), hydrogen fluoride (HF), and phosphoricacid (H3PO4), among others, and may be performed to remove a bulkportion of the first semiconductor wafer/substrate 100/110 down to thefunctional TSVs 160 and the dummy TSVs 210. For example, where thesubstrate 110 has a thickness of about 700 microns, the bulk thinningmay remove between about 600 microns and about 670 microns of thesubstrate 110, such as about 650 microns of the substrate 110, althoughother removal thicknesses are also within the scope of the presentdisclosure. One or more subsequent over-etch processes may then remove aportion of the barrier layer(s) 162 and perhaps portions of protrudingends of the functional TSVs 160 and the dummy TSVs 210. Suchover-etching may utilize an etchant such as tetramethylammoniumn-hydroxide (TMAH) and/or other etchants having a high selectivitybetween the material of the substrate 110 (e.g., silicon) and theconductive material 164 of the functional TSVs 160 and the dummy TSVs210 (e.g., copper). Such over-etching may remove the material of thesubstrate 110 such that the functional TSVs 160 and the dummy TSVs 210protrude from the substrate 110 a distance D, which may range betweenabout 0.5 microns and about 10 microns, such as about 1 micron, althoughother protrusion distances are also within the scope of the presentdisclosure.

FIG. 3 is a sectional view of the apparatus shown in FIG. 2 in asubsequent stage of manufacture in which an insulating layer 310 hasbeen formed over the back side 104, as well as the protruding ends ofthe functional TSVs 160 and the dummy TSVs 210. The insulating layer 310may comprise one or more layers of dielectric materials, such as siliconnitride, silicon oxide, and/or other materials, which may be formedutilizing CVD, PECVD, MOCVD, PVD, sputtering, and/or other processes.The insulating layer 310 may have a thickness ranging between about 10microns and about 100 microns, such as a thickness of about 45 microns.

FIG. 4 is a sectional view of the apparatus shown in FIG. 3 in asubsequent stage of manufacture in which one or more CMP and/or otherplanarizing processes have been performed to remove portions of theinsulating layer 310 from over the protruding ends of the functionalTSVs 160 and the dummy TSVs 210. The planarizing may be performed longenough so that the top surface 166 of the functional TSV 160 issubstantially level with or protrudes from surrounding surfaces (e.g.,of layer 310) by a distance P1, which may range between 0 and about 1000Angstroms. However, perhaps as a result of the number density of thedummy TSVs 210, the dummy TSVs 210 may protrude from surroundingsurfaces (along with portions of the insulating layer 310) by a distanceP2, which may range between about 1000 Angstroms and about 4000Angstroms.

FIG. 5 is a sectional view of the apparatus shown in FIG. 4 in asubsequent stage of manufacture in which a redistribution layer (RDL)510 has been formed over the insulating layer 310, the functional TSVs160, and the dummy TSVs 210. The RDL 510 may comprise one or more layersof conductive materials, such as aluminum, gold, silver, titanium,titanium nitride, tantalum nitride, titanium, and/or other materials,which may be formed utilizing CVD, PECVD, MOCVD, PVD, sputtering, and/orother processes. The RDL 510 may have a thickness ranging between about10 microns and about 100 microns, such as a thickness of about 45microns. As a result, a step height P3 of the RDL 510 over thefunctional TSV 166 may range between 0 and about 1000 Angstroms, whereasa step height P4 of the RDL 510 over the dummy TSVs 210 may rangebetween about 1000 Angstroms and about 4000 Angstroms.

FIG. 6 is a sectional view of the apparatus shown in FIG. 5 in asubsequent stage of manufacture in which one or more CMP and/or otherplanarizing processes have been performed to remove portions of the RDL510 from over the dummy TSVs 210 and at least partially remove the RDL510 from over the functional TSVs 160. For example, planarizing may beperformed so that a top surface 512 of the RDL 510 over the functionalTSV 160 protrudes from surrounding surfaces (e.g., of RDL 510) to a stepheight P5, or so that the top surface 168 of the functional TSV 160 isexposed and is substantially level with or protrudes from surroundingsurfaces (e.g., of RDL 510) to the step height P5. The step height P5may range between 0 and about 1000 Angstroms. Top surfaces 212 of thedummy TSVs 210 may simultaneously be exposed, and may protrude fromsurrounding surfaces (along with portions of the insulating layer 310and/or redistribution layer 510) to a step height P6, which may rangebetween about 1000 Angstroms and about 4000 Angstroms.

For example, the step heights P5 over the functional TSVs may be lessthan a predetermined value, and the step heights P6 over the dummy TSVsmay be greater than the predetermined value. The predetermined value maybe about 1000 Angstroms, although others are also within the scope ofthe present disclosure. In some implementations, the step heights P5 maybe less than about 700 Angstroms and the step heights P6 may be greaterthan about 3000 Angstroms.

Some or all of the step heights P5 may each be a height differencebetween a planarized portion 512 of the RDL 510 and an un-planarizedportion of the RDL 510. Some or all of step heights P5 may alternativelybe a height difference between a portion of the RDL 510 and the exposedend 168 of one or more of the functional TSVs 160. The step heights P6may each be a height difference between a portion of the RDL and theexposed ends 212 of one or more of the dummy TSVs 210.

FIG. 7 is a plan view of the backside 104 of the apparatus shown in FIG.6, demonstrating how the functional TSVs 160 are located within dieareas 106 while the dummy TSVs 210 are located within the die streets108. As also depicted in FIG. 7, the dummy TSVs 210 may be grouped forusage as alignment marks during manufacturing, such as a rectangularalignment mark 710, a triangular alignment mark 720, and/or across-shaped alignment mark 730, as well as other alignment marks 740that may have a shape representing a combination of these and otherbasic geometric shapes.

One or more functional TSVs 160 and one or more dummy TSVs 210 maycollectively be located within a die area 106 surrounded by a die street108. One such example depicted in FIG. 7 comprises a rectangularalignment mark 750 formed by the dummy TSVs. In other implementations,each of the functional TSVs 160 may be located within the die areas 106and each of the dummy TSVs 210 may be located within the die street 108.In other implementations, each of the functional TSVs 160 may be locatedwithin the die areas 106, a first subset of the dummy TSVs 210 may belocated within one or more of the die areas 106, and a second subset ofthe dummy TSVs 210 may be located within one or more of the die streets108.

As also shown in FIG. 7, the number/area or “density” of the dummy TSVs210 may be substantially greater than the density of the functional TSVs160. Thus, the dummy TSVs 210 may be located densely, and the functionalTSVs 160 may be located spaced apart.

For example, a closest two of the functional TSVs 160 may be laterallyoffset by a first distance L1, and a closest two of the dummy TSVs 210may be laterally offset by a second distance L2. The first distance L1may be at least twice, triple, or quadruple the second distance L2, ifnot more. The first distance L1 may be at least about 100 microns, andthe second distance L2 may be less than about 30 microns. In suchimplementations, and others within the present scope, the functionalTSVs 160 and the dummy TSVs 210 may have a diameter ranging betweenabout 10 microns and about 15 microns, such as about 12 microns.

FIG. 8 is a sectional view of the apparatus shown in FIG. 6 in asubsequent stage of manufacture in which a second semiconductor die 806has been bonded to the first semiconductor die 106. The secondsemiconductor die 806 may comprise structures similar to the firstsemiconductor die 106, such as a second substrate 810, a second set ofactive devices 820, a second interconnect structure 830, and secondcontact bumps 850, some or all of which may be formed in a similarfashion as described above with respect to FIG. 1. However, the secondsemiconductor die 806 is not intended to be limited to the exampledepicted in FIG. 8, and any suitable device may alternatively beconnected to the first semiconductor die 106. Assembling the secondsemiconductor die 806 to the first semiconductor die 106 may utilize theabove-described dummy TSVs 210 and/or one or more of the alignment marks710, 720, 730, and 740 (not shown in FIG. 8 for purposes of clarity).

Bonding the first semiconductor die 106 to the second semiconductor die806 may comprise aligning the second contact bumps 850 with thefunctional TSVs 160 of the first semiconductor die 106, perhapsutilizing one or more of the alignment marks depicted in FIG. 7. Oncealigned, the second contact bumps 850 and the functional TSVs 160 may bebonded together by contacting the second contact bumps 850 to thefunctional TSVs 160 and performing a reflow to reflow the material ofthe second contact bumps 850 and bond to the functional TSVs 160.However, copper-copper bonding and/or one or more other bonding methodsmay alternatively or additionally be utilized to bond the secondsemiconductor die 806 to the first semiconductor die 106.

An underfill material 890 may be injected or otherwise formed in thespace between the two dies 106 and 806. The underfill material 890 maybe or comprise a liquid epoxy, for example, which may be dispensedbetween the two dies 106 and 806 and then cured to harden. The underfillmaterial 890 may aid in preventing cracks from forming in the contactbumps 850, such as may otherwise be caused by thermal and/or otherstresses. A deformable gel and/or silicon rubber may also oralternatively be formed between the two dies 106 and 806. A saw bladeand/or other means may then be utilized to slice the dies 106 and 806along the die streets, although other singulation processes may also oralternatively be performed.

The foregoing outlines features of several embodiments so that a personhaving ordinary skill in the art may better understand the aspects ofthe present disclosure. A person having ordinary skill in the art shouldappreciate that they may readily use the present disclosure as a basisfor designing or modifying other processes and structures for carryingout the same purposes and/or achieving the same advantages of theembodiments introduced herein. A person having ordinary skill in the artshould also realize that such equivalent constructions do not departfrom the spirit and scope of the present disclosure, and that they maymake various changes, substitutions and alterations herein withoutdeparting from the spirit and scope of the present disclosure.

1. An apparatus, comprising: a substrate comprising a plurality ofelectronic devices; an interconnect structure formed on a first side ofthe substrate and interconnecting ones of the plurality of electronicdevices; a plurality of dummy through-silicon-vias (TSVs) each extendingthrough the substrate and forming an alignment mark on a second side ofthe substrate; a plurality of functional TSVs each extending through thesubstrate and electrically connected to one of the plurality ofelectronic devices; and a redistribution layer (RDL) formed on thesecond side of the substrate and interconnecting ones of the pluralityof dummy TSVs with ones of the functional TSVs, wherein first stepheights over the functional TSVs are less than a predetermined value,and wherein second step heights over the dummy TSVs are greater than thepredetermined value.
 2. The apparatus of claim 1 wherein thepredetermined value is about 1000 Angstroms.
 3. The apparatus of claim 1wherein the first step heights are less than about 700 Angstroms, andwherein the second step heights are greater than about 3000 Angstroms.4. The apparatus of claim 1 wherein the first step heights are each aheight difference between a planarized portion of the RDL and anun-planarized portion of the RDL.
 5. The apparatus of claim 1 whereinthe first step heights are each a height difference between a portion ofthe RDL and an exposed surface of one of the functional TSVs.
 6. Theapparatus of claim 1 wherein the second step heights are each a heightdifference between a portion of the RDL and an exposed surface of one ofthe dummy TSVs.
 7. The apparatus of claim 1 wherein the RDLsubstantially comprises aluminum.
 8. The apparatus of claim 1 whereineach of the functional TSVs and each of the dummy TSVs are locatedwithin a die area surrounded by a die street.
 9. The apparatus of claim1 wherein each of the functional TSVs are located within a die areasurrounded by a die street, and wherein each of the dummy TSVs arelocated within the die street.
 10. The apparatus of claim 1 wherein aclosest two of the functional TSVs are laterally offset by a firstdistance, wherein a closest two of the dummy TSVs are laterally offsetby a second distance, and wherein the first distance is at least twicethe second distance.
 11. The apparatus of claim 10 wherein the firstdistance is at least about 100 microns, and wherein the second distanceis less than about 30 microns.
 12. The apparatus of claim 1 wherein thealignment mark is selected from the group consisting of: a cross-shapedalignment mark; a triangular alignment mark; a rectangular alignmentmark; a combination alignment mark having a shape formed by acombination of two or more of: a cross-shape; a triangular shape; and arectangular shape. 13-16. (canceled)
 17. An apparatus, comprising: asubstrate comprising a plurality of electronic devices; an interconnectstructure formed on a first side of the substrate and interconnectingones of the plurality of electronic devices; a plurality of firstthrough-silicon-vias (TSVs) each extending through the substrate andelectrically connected to one of the plurality of electronic deviceswithin a die area surrounded by a die street; a plurality of second TSVseach extending through the substrate and forming an alignment markwithin the die area; and a redistribution layer (RDL) formed on a secondside of the substrate and interconnecting ones of the pluralities offirst and second TSVs.
 18. The apparatus of claim 17 wherein first stepheights over the first TSVs are less than a predetermined value, andwherein second step heights over the second TSVs are greater than thepredetermined value.
 19. The apparatus of claim 17 wherein a closest twoof the first TSVs are laterally offset by a first distance, wherein aclosest two of the second TSVs are laterally offset by a seconddistance, and wherein the first distance is at least twice the seconddistance.
 20. The apparatus of claim 17 wherein the alignment mark isselected from the group consisting of: a cross-shaped alignment mark; atriangular alignment mark; a rectangular alignment mark; a combinationalignment mark having a shape formed by a combination of two or more of:a cross-shape; a triangular shape; and a rectangular shape.
 21. Anapparatus, comprising: a plurality of integrated circuit elements on asubstrate; an interconnect structure formed over the integrated circuitelements and comprising one or more metal layers embedded in respectivedielectric layers; a plurality of dummy through-silicon-vias (TSVs) eachextending through the substrate; a plurality of functional TSVs eachextending through the substrate and electrically connected to one ormore of the integrated circuit elements; and a redistribution layer(RDL) formed on a side of the substrate opposite the side on which isformed the interconnect structure, and interconnecting ones of theplurality of dummy TSVs with ones of the functional TSVs, wherein firststep heights over the functional TSVs are less second step heights overthe dummy TSVs.
 22. The apparatus of claim 21, wherein the first stepheights are each a height difference between a portion of the RDL and anexposed surface of one of the functional TSVs and wherein the secondstep heights are each a height difference between a portion of the RDLand an exposed surface of one of the dummy TSVs.
 23. The apparatus ofclaim 21 wherein each of the functional TSVs and each of the dummy TSVsare located within a die area surrounded by a die street.
 24. Theapparatus of claim 21 wherein a closest two of the functional TSVs arelaterally offset by a first distance, wherein a closest two of the dummyTSVs are laterally offset by a second distance, and wherein the firstdistance is at least twice the second distance.